Download Population Genetics and Evolution LAB 8A

Population Genetics and Evolution
LAB 8A- PTC
Introduction:
In 1908, G.H.Hardy and W. Weinberg independently suggested a scheme whereby evolution
could be viewed as changes in frequency of alleles in a population of organisms. In this scheme,
if A and a are alleles for a particular gene locus and each diploid individual has two such loci,
then p can be designated as the frequency of the A allele and q as the frequency of the a allele.
For example, in a population of 100 individuals (each with two loci) in which 40% of the alleles
are A, p would be 0.40. The rest of the alleles would be (60%) would be a and q would be equal
to 0.60. p + q = 1. These are referred to as allele frequencies. The frequency of the possible
diploid combinations of these alleles (AA, Aa, aa) is expressed as p2 +2pq +q2 = 1.0. Hardy and
Weinberg also argued that if 5 conditions are met, the population's alleles and genotype
frequencies will remain constant from generation to generation. These conditions are as follows:
The breeding population is large. (Reduces the problem of genetic drift.)
Mating is random. (Individual show no preference for a particular mating type.)
There is no mutation of the alleles.
No differential migration occurs. (No immigration or emigration.)
There is no selection. (All genotypes have an equal chance of surviving and
reproducing.)
The Hardy-Weinberg equation describes an existing situation. Of what value is such a rule?
It provides a yardstick by which changes in allelic frequencies can be measured. If a
population's allelic frequencies change, it is undergoing evolution.
Background:
Estimating Allele Frequencies for a Specific Trait within a Sample Population:
Using the class as a sample population, the allele frequency of a gene controlling the ability
to taste the chemical PTC (phenylthiocarbamide) could be estimated. A bitter taste reaction is
evidence of the presence of a dominant allele in either a homozygous (AA) or heterozygous (Aa)
condition. The inability to taste the PTC is dependent on the presence of the two recessive alleles
(aa). To estimate the frequency of the PTC -tasting allele in the population, one must find p. To
find p, one must first determine q (the frequency of the non tasting allele). PTC trait has been
shown to correlate with a number of dietary preferences and thus may have important
implications for human health. This gene is a member of the family of bitter taste receptor
genes. It exists in seven different allelic forms, although only two of these, designated the major
taster and major non- taster forms, exist at high frequency. The two major forms differ from
each other at three amino acid positions.
LAB 8A- PTC Procedure:
1. Using the PTC taste test paper, tear off a short strip and press it to your tongue tip. PTC
tasters will sense a bitter taste.
2. A decimal number representing the frequency of tasters (p2+2pq) should be calculated by
dividing the number of tasters in the class by the total number of students in the class. A
decimal number representing the frequency of the non tasters (q2) can be obtained by
dividing the number of non tasters by the total number of students. You should then
record these numbers in Table 8.1.
3. Use the Hardy-Weinberg equation to determine the frequencies (p and q) of the two
alleles. The frequency q can be calculated by taking the square root of q 2. Once q has
been determined, p can be determined because 1-q=p. Record these values in Table 8.1
for the class and also calculate and record values of p and q for the North American
population.
Table 8.1 Phenotypic Proportions of Tasters and Nontasters and Frequencies of the
Determining Alleles
Phenotypes
Allele Frequency Based on
the H-W Equation
2
2
Tasters (p +2pq)
Non Tastes(q )
p
q
#
%
#
%
Class
Population
North American
Population
0.55
0.45
Discussion Questions:
1. What is the percentage of heterozygous tasters (2pq) in your class?
2. What percentage of the North American population is heterozygous for the taster allele?
3. Do you think that if we tested the North American population for the PTC allele in
several generations that the population would be in Hardy Weinberg equilibrium? Why
or why not?
4. What would be an evolutionary advantage of being dominant for the taster allele?
5. What would be an evolutionary disadvantage of being dominant for the taster allele?
AP Biology LAB 8B- Case Studies:
Case 1- Test of an Ideal Hardy-Weinberg Community
The entire class will represent a breeding population. The class will simulate a
population of randomly mating heterozygous individuals with an initial gene frequency of 0.5 for
the dominant allele A and the recessive allele a and genotype frequencies of 0.25 AA, 0.50 Aa,
and 0.25 aa. Record this on the Data page at the end of the lab. Each member of the class will
receive four cards. Two cards will have A and two cards will have a. The four cars represent the
products of meiosis. Each "parent" will contribute a haploid set of chromosomes to the next
generation.
Procedure:
1. Creating F1- Choose another student at random (to ensure random mating). Assume that
gender and genotype are irrelevant to mate selection.
2. Turn the four cards over so the letters are not showing, shuffle them, and take the card on
top to contribute to the production of the first offspring. Your partner should do the same.
3. Put the cards together. The two cards represent the alleles of the first offspring (F1
genotype). One of you should record the genotype of this offspring in the Case 1 of the
Data Page. Each student pair must produce two offspring (one for each person), so all
four cards must be reshuffled and the process repeated to produce a second offspring for
the other person to record.
4. Creating F2- Each student should obtain, if necessary, new cards representing their alleles
in his or her respective gametes after the process of meiosis. For example, if student 1
becomes the genotype Aa then obtain cards A, A, a, a; if student 2 becomes aa then
obtain cards a, a, a, a.
5. Each participant should randomly seek out another person with whom to mate in order to
produce offspring of the next generation. You should follow the same mating procedure
as for the first generation, being sure you record your new genotype after each generation
in the Case 1 section.
6. Class data should be collected after each generation for five generations. At the end of
each generation, remember to record the genotype that you have assumed. Your teacher
will collect class data after each generation by asking you to raise your hand to report
your genotype.
7. Calculate the allele frequencies, p and q, for the population after five generations of
simulated random mating. Total # of alleles in the population = total # students x 2.
# of A alleles at the fifth generation
# of a alleles at the fifth generation
# of AA offspring _____ X 2= _____ A alleles
# of Aa offspring _____ X 1= ______A alleles
# of aa offspring ______ X 2= _______ a alleles
# of Aa offspring ______ X 1= _______a alleles
Total = ____________ A alleles
Total = ____________ a alleles
p = Total # of A alleles ____ =
Total # of alleles in the population
q = Total # of a alleles
____ =
Total # of alleles in the population
LAB 8B- Case 2- Selection
In this case you will modify the simulation to make it more realistic. In the natural environment,
not all genotypes have the same rate of survival; that is, the environment might favor some
genotypes while selecting against others. An example is the human condition sickle-celled
anemia. It is a condition caused by a mutation on one allele, in which a homozygous recessive
does not survive to reproduce. For this simulation, you will assume that the homozygous
recessive individuals never survive. Heterozygous and homozygous dominant individuals
always survive.
Procedures:
1. Start again with your initial genotype, and produce your "offspring" as in Case 1. Every
time your offspring is aa it does not reproduce. Since we want to maintain a constant
population size, the same two parents must try again until they produce two surviving
offspring.
2. Proceed through five generations, selecting against the homozygous offspring 100% of
the time. Then add up the genotype frequencies that exist in the population and calculate
the new p and q frequencies in the same way as it was done in Case 1.
# of A alleles at the fifth generation
# of a alleles at the fifth generation
# of AA offspring ____ X 2= ____ A alleles
# of Aa offspring ____ X 1= _____A alleles
# of aa offspring _____ X 2= _____ a alleles
# of Aa offspring _____ X 1= ____a alleles
Total = ____________ A alleles
Total = ____________ a alleles
p = Total # of A alleles ____ =
Total # of alleles in the population
q = Total # of a alleles
____ =
Total # of alleles in the population
1. How do the new frequencies of p and q compare to the initial frequencies in Case 1?
2. How has the allelic frequency of the population changed?
3. Predict what would happen to the frequencies of p and q if you simulated another 5
generations.
4. In a large population, would it be possible to completely eliminate a deleterious recessive
allele? Explain.
LAB 8B- Case 3- Sickle Cell
This case simulates sickle-cell anemia in human genomes. Just like in CASE 2, the homozygous
recessive, aa, alleles never survive. Individuals that are heterozygous to the disease are slightly
more resistant than individuals who are homozygous dominant. With this in mind, if you draw
up AA, then you must flip a coin to see whether or not the offspring will survive or not, heads
survives, tails dies. You must always try to get 2 offspring, no matter how long it takes.
# of A alleles at the fifth generation
# of a alleles at the fifth generation
# of AA offspring ____ X 2= _____ A alleles
# of Aa offspring _____ X 1= ______A alleles
# of aa offspring _____ X 2= ______ a alleles
# of Aa offspring _____ X 1= ______a alleles
Total = ____________ A alleles
Total = ____________ a alleles
p = Total # of A alleles ____ =
Total # of alleles in the population
q = Total # of a alleles
____ =
Total # of alleles in the population
Questions:
1. Explain how the changes in p and q frequencies in Case 2 compare with Case 1 and Case 3.
2. Will the recessive allele will be completely eliminated in either Case 2 or Case 3? Explain.
3. What is the importance of heterozygotes in maintaining genetic variation in populations
LAB 8B- Case 4- Genetic Drift
The case is just like CASE I except after each generation is produced, there will be no random
selection of new mates. The purpose of this case is to simulate isolated populations and genetic
drift. Each group must not interact in any way in order to represent isolated populations. Record
the genotypes after each generation and calculate the frequencies for p and q as usual.
# of A alleles at the fifth generation
# of a alleles at the fifth generation
# of AA offspring ____ X 2= ____ A alleles
# of Aa offspring ____ X 1= _____A alleles
# of aa offspring _____ X 2= _____ a alleles
# of Aa offspring _____ X 1= ____a alleles
Total = ____________ A alleles
Total = ____________ a alleles
p = Total # of A alleles ____ =
Total # of alleles in the population
q = Total # of a alleles
____ =
Total # of alleles in the population
1. What do your results indicate about the importance of population size as an evolutionary
force?
Data Page:
Case 1- Hardy-Weinberg Equilibrium
Initial Class Frequencies:
AA _______ Aa_______ aa________
My initial genotype :_______________
F1 Genotype ______
F2 Genotype ______
F3 Genotype ______
F4 Genotype ______
F5 Genotype ______
Final Class Frequencies:
AA _______ Aa_______ aa________
p _________ q __________
Case 2- Selection (aa=death)
Initial Class Frequencies:
AA _______ Aa_______ aa________
My initial genotype :_______________
F1 Genotype ______
F2 Genotype ______
F3 Genotype ______
F4 Genotype ______
F5 Genotype ______
Final Class Frequencies:
AA _______ Aa_______ aa________
p _________ q __________
Case 3- Heterozygote Advantage
Initial Class Frequencies:
AA _______ Aa_______ aa________
My initial genotype :_______________
F1 Genotype ______ F6 Genotype ______
F2 Genotype ______ F7 Genotype ______
F3 Genotype ______ F8 Genotype ______
F4 Genotype ______ F9 Genotype ______
F5 Genotype ______ F10 Genotype ______
Final Class Frequencies: (after five generations)
AA _______ Aa_______ aa________
p _________ q __________
Final Class Frequencies: (after ten generations)
AA _______ Aa_______ aa________
p _________ q __________
Case 3- Genetic Drift
Initial Class Frequencies:
AA _______ Aa_______ aa________
p _____ q ______
My initial genotype :_______________
F1 Genotype ______
F2 Genotype ______
F3 Genotype ______
F4 Genotype ______
F5 Genotype ______
Final Class Frequencies:
AA _______ Aa_______ aa________
p _________ q __________
Extension:
Hardy-Weinberg Problems (show ALL work)
1. In Drosophila, the allele for normal length wings is dominant over the allele for vestigial
wings. In a population of 1,000 individuals, 360 show the recessive phenotype. How many
individuals would you expect to be homozygous dominant and heterozygous for this trait?
2. The allele for the ability to roll one's tongue is dominant over the allele for the lack of this
ability. In a population of 500 individuals, 25% show the recessive phenotype. How many
individuals would you expect to be homozygous dominant and heterozygous for this trait?
3. The allele for the hair pattern called "widow's peak" is dominant over the allele for no
"widow's peak." In a population of 1,000 individuals, 510 show the dominant phenotype. How
many individuals would you expect of each of the possible three genotypes for this trait?
4. In a certain population, the dominant phenotype of a certain trait occurs 91 % of the time.
What is the frequency of the dominant allele?